The present invention relates to orthopedic instrumentation including external fixation devices and in particular to a method and apparatus for the clinical use of load measurements taken during distraction osteogenesis.
Distraction osteogenesis is a technique for lengthening a bone so as to minimize differences in the lengths of a patient's limbs or for similar purposes. Professor G. A. Ilizarov developed a technique for bone lengthening using the technique of distraction osteogenesis which employs an external fixator attached to the patient's bone by tensioned small diameter wires. The Ilizarov technique may be used for various orthopedic purposes other than bone distraction. A description of the Ilizarov technique may be found in U.S. Pat. No. 4,615,338 issued to Ilizarov, et al. on Oct. 7, 1986 for "Automatic Compression-Distraction Apparatus", incorporated herein by reference.
Typically the Ilizarov apparatus employs two or more fixator rings which may be either complete or partial rings. Transosseous wires and/or pins are fixed to the fixator rings at each of their ends by means of wire fixation bolts after being passed through holes drilled in the patient's bone and through the soft tissue of the limb. In the basic form of the Ilizarov apparatus, distraction rod assemblies are attached to the fixator rings. The distraction rod assemblies comprise threaded rods and telescopic rods into which the threaded rods are received. The distraction rod assemblies may be fastened to the fixator rings using nuts or knobs which may be turned to gradually lengthen the distraction rod assemblies so as to force the fixator rings further apart.
For distraction osteogenesis of a leg bone the Ilizarov apparatus is used in the following manner. A site is selected on the bone to be lengthened. The external fixator rings are attached to the patient's leg on either side of the selected site by means of the transosseous wires which are passed through the soft tissue and bone of the patient's limb. The transosseous wires are attached at their opposite ends to the respective fixator rings. A corticotomy is performed through a small incision in the patient's limb. The patient's bone is separated at the selected site while preserving the blood supply within the interior of the bone and with minimal damage to the soft tissue in the vicinity of the severed bone. Following surgery, nuts on the distraction rods may be tightened to force the fixator rings apart and thereby applying tension to the tissues in the biological zone between the bony surfaces at the corticotomy site. Bony regeneration at the surface of the severed bone begins to occur as the process of distraction continues. The typical rate of distraction is approximately 1 mm per day until the necessary length of regenerated bone is achieved. Afterwards the external fixator is kept in place until the newly generated bone has stabilized.
While distraction osteogenesis is typically performed by manual adjustment of the rate of distraction, systems have been suggested for automatic distraction. U.S. Pat. No. 5,180,380 issued to Pursley, et al. on Jan. 19, 1993 for "Automatic Compression-Distraction-Torsion Method and Apparatus" discloses a system which includes at least one motor for incrementally adjusting the distraction rods.
U.S. Pat. No. 5,108,394 issued to Kurokawa, et al. on Apr. 28, 1992 for "Bone Fixing Device" discloses a bone fixing device comprising a single rod member and an axial actuator. Kurokawa furthermore discloses a load cell for detecting what is described as a repulsive force from the bone.
Various attempts have been made to understand the biomechanical forces involved in the distraction osteogenesis procedure. However, the studies to date have simply focused on the nature of the loads generated during the distraction process without attempting to apply that information in a clinical setting to improve the clinical outcome. Prior to the development of the method and apparatus of the present invention, distraction has been carried out at a fixed rate regardless of the loads generated in the patient during the process. It has been discovered that a knowledge of the loads generated by the distraction procedure across the active biological zone of the corticotomy site may be used to adjust the rate of distraction to avoid unsuccessful clinical outcomes. An unsuccessful clinical outcome could include either premature consolidation of the biological bridge between the surfaces of the severed bone or nonunion which is a result of a disruption of the biological bridge. In addition, the information derived from the measurement of loads at the corticotomy site may be used to test the degree of bony consolidation at the corticotomy site and thus to determine when healing has progressed to the point that the fixator may be removed from the patient.
Furthermore, prior to the development of the method and apparatus of the present invention, no consideration has been given to the variation in cross-sectional area of the bone in various patients. As a result there has been no recognition of the variation in stress or load per unit area in various patients in which the loads alone would appear to be otherwise similar. The method and apparatus of the present invention recognizes that an improved clinical outcome may be obtained by converting measured load information into stress values by comparing the measured loads to the measured cross-sectional area of the corticotomy site. The use of such derived stress information may then be applied to adjust the rate of distraction so as to avoid unsuccessful clinical outcomes.
Other objects, advantages and features of the present invention will be more fully understood when considered in conjunction with the following detailed description of the preferred embodiments together with the drawings.